Conventionally, in source gas supply devices for semiconductor manufacturing devices, etc., heat-type flow control devices and pressure-type flow control devices have been widely used for controlling the supplied gas flow. In particular, the latter pressure-type flow control device FCS is, as shown in FIG. 5, composed of a control valve CV, a temperature detector T, a pressure detector P, an orifice OL, a computation control unit CD, and the like. Such a flow control device has the excellent flow characteristic that stable flow control can be achieved even when the primary-side supply pressure is significantly changed.
That is, in the pressure-type flow control device FCS of FIG. 5, the computation control unit CD is composed of a temperature correction/flow computation circuit CDa, a comparison circuit CDb, an input/output circuit CDc, an output circuit CDd, and the like. Then, values detected at the pressure detector P and the temperature detector T are input to the temperature correction/flow computation circuit CDa, where the detected pressure is subjected to temperature correction and flow computation, and then the flow computation value Qt is input to the comparison circuit CDb.
In addition, an input signal Qs corresponding to the set flow is input from the terminal In and then input to the comparison circuit CDb through the input/output circuit CDc, where the signal is compared with the flow computation value Qt from the temperature correction/flow computation circuit CDa described above. When the comparison shows a difference between the set flow input signal Qs and the flow computation value Qt, a control signal Pd is output to the actuator of the control valve CV. Accordingly, the control valve CV is actuated, and automatic regulation is performed to make the difference between the set flow input signal Qs and the computed flow value Qt (Qs−Qt) become zero.
In the pressure-type flow control device FCS described above, when a so-called critical expansion condition of P1/P2≧about 2 is maintained between the pressure P2 on the downstream side and the pressure P1 on the upstream side of the orifice OL, the gas flow Q through the orifice OL is represented by Q=KP1 (wherein K is a constant). Meanwhile, when the critical expansion condition is not satisfied, the gas flow Q through the orifice OL is calculated as Q=KP2m(P1−P2)n (wherein K, m, and n are constants).
This makes it possible to exert the excellent characteristics that the flow Q can be controlled with high accuracy by controlling the pressure P1, and further, even when there is a significant change in the pressure of the gas Go on the upstream side of the control valve CV, the controlled flow value hardly changes.
The pressure-type flow control device designed such that the gas flow Q is computed as Q=KP1 (wherein K is a constant) as described above is generally called FCS-N type. In addition, the pressure-type flow control device designed such that the gas flow Q is computed as Q=KP2m(P1−P2)n (wherein K, m, and n are constants) is called FCS-WR type.
As pressure-type flow control devices of this kind, other types exist, including those in which an orifice of the above FCS-N type is configured as a plurality of orifices OL connected in parallel, and they are arbitrarily selected by a switching valve to allow the flow control range to be changed (FCS-SN type), and those in which the same orifice mechanism is used as an orifice of the above FCS-WR type (FCS-SWR type).
FIG. 6 shows configuration system diagrams of the above FCS-N type (JP-A-8-338546, etc.), FCS-SN type (JP-A-2006-330851, etc.), FCS-WR type (JP-A-2003-195948, etc.), and FCS-SWR type (Patent Application No. 2010-512916, etc.). Their configurations, operation principles, and the like are already known, and thus the detailed description thereof will be omitted herein.
In FIG. 6, reference signs P1 and P2 denote pressure sensors, a reference sign CV denotes a control valve, a reference sign OL denotes an orifice, a reference sign OL1 denotes a small-diameter orifice, a reference sign OL2 denotes a large-diameter orifice, and a reference sign ORV denotes an orifice switching valve.
When the source gas is supplied, it is possible to supply a predetermined amount of source gas using a pressure-type flow control device FCS of this kind. However, in order to supply the source gas more precisely, it has been demanded to perform so-called pulse control with improved flow step-up and step-down characteristics.
FIG. 7 is a configuration system diagram of a pressure-type flow control device capable of such pulse control, in which the internal volume between an orifice OL and an on/off valve Vp is minimized to offer excellent step-up and step-down characteristics, making it possible to perform pulse control with high accuracy.
However, in a conventional pressure-type flow control device in which an on/off valve is provided on the orifice downstream side and pulse flow control is achieved by controlling the valve to open or close, the internal pressure of the fluid channel may increase due to minute leakage of the source gas from the control valve while the control valve CV is closed to stop the flow control. This results in a problem in that when the flow control is re-started, due to the increased internal pressure of the fluid channel, the controlled flow value “overshoots” at the time of step-up.